It is generally accepted that Okazaki pieces, polydeoxyribonucleotide chains of about 500 - 1000 nucleotides in length, are intermediates in the synthesis of the high molecular weight chromosomal DNA. The mechanism of synthesis of Okazaki pieces, however, especially their initiation, is still little understood. Recent results from our laboratory show that most of the polymerization of nucleotides in HeLa cells occurs at the oligonucleotide level (Jering and Werner, 1976). Together with the exponential size distribution of nascent DNA chains (Kuebbing et al., 1976) these results suggest that longer chains arise primarily by ligation of oligonucleotides with little further incorporation of mononucleotides. In addition, we are studying the nature of proteins that are non-covalently linked to all nascent DNA chains. It is hoped that this investigation will contribute to our understanding of the replication process.